The invention relates to control systems for electrohydraulic steering controllers for work vehicles, such as agricultural and construction vehicles. More particularly, it relates to such control systems having fuzzy logic control elements.
Agricultural vehicles are designed to travel on fields performing desired agricultural operations. For safe and productive operations, one of the fundamental requirements for agricultural vehicles is high maneuverability. The adoption of electrohydraulic (E/H) steering on agricultural vehicles makes it possible to apply electronic control for better vehicle maneuverability. However, the highly nonlinear nature of the E/H system will affect the performance of the vehicle steering. Variations in field conditions make it more difficult to have high performance steering control. Conventional control technologies have limitations in solving such problems effectively (Qiu, et al. 1999; Wu, et al. 1998), yet a skillful operator adapts well to such changing conditions.
Fuzzy control is an advanced control technology that can mimic a human operator""s operating strategy to control a complicated system (Pedrycz 1993), and can handle systems with uncertainty and nonlinearity (Yen, et al. 1994; Corbet, et al. 1996). Numerous fuzzy control applications on vehicles have been reported. Todo, et al. (1999) developed a fuzzy controller that utilized the offset and the orientation errors to control the steering of a mobile robot and resulted in satisfactory trajectory tracking performance. Zhang, et al. (1999) developed an adaptive fuzzy controller for a One Degree-of-Freedom (1-DOF) E/H actuator system. This fuzzy controller compensated for system non-linearities and provided accurate velocity control on the hydraulic actuator while subjected to changing load.
What is needed is a control system for an E/H steering system that compensates for these inherent non-linearities. This application presents the development and construction of a fuzzy steering controller for agricultural and construction vehicles with E/H steering systems that compensates for these non-linearities. The controller steers a vehicle to follow desired trajectories based upon the steering rate command and the error in steering angle.
In accordance with a first embodiment of the invention, a steering controller for a wheeled work vehicle is provided having a hydraulic actuator coupled to steerable wheels on the vehicle and driven by a proportional control electrohydraulic valve, a sensor that generates a signal indicative of the degree of turning of the wheels and the actual position of the wheels, the controller including a first fuzzifying circuit configured to convert a single real-valued turning rate signal into a first set of two values indicative of membership in two fuzzy sets, a second fuzzifying circuit configured to convert a single real-valued turning position signal into a second set of two values indicative of membership in two fuzzy sets, a fuzzy rule base including a plurality of rules to convert the first and second sets of values into a third set of values indicating membership in two or more fuzzy steering command sets, a fuzzy inference engine configured to apply the plurality of rules to the first and second sets and derive the third set of values; and a defuzzifying circuit configured to combine the third set of values into a single real-valued steering command indicative of the degree of opening of the proportional control electrohydraulic valve.
The first set may represent the degree of membership in two of at least five fuzzy domains, including one domain representative of a steering rate of zero. The first set may represent the degree of membership in two of at least seven fuzzy domains. The second set may represent the degree of membership in two of at least five fuzzy domains, including at least one domain representative of a steering error of zero degrees. The second set may represent the degree of membership in two of at least seven fuzzy domains. The third set may represent the degree of membership in two of at least five fuzzy domains, including at least one domain representative of a steering command indicative of zero flow through the valve. The third set may represent the degree of membership in two of at least seven fuzzy domains.
In accordance with a second embodiment of the invention a wheeled work vehicle is provided, including a chassis, a plurality of steerable wheels pivotally mounted on the chassis, at least one hydraulic actuator coupled to the plurality of steerable wheels and to the chassis to steer the wheels left and right with respect to the chassis, an electrohydraulic proportional control valve fluidly coupled to the at least one actuator to regulate the flow of hydraulic fluid to the actuator in proportion to a valve signal, a driver circuit configured to convert a steering command signal to the valve signal, a microprocessor coupled to the driver circuit and configured to calculate the steering command signal based at least upon a signal indicative of an error in wheel steering position and a signal indicative of a rate of change of wheel steering position, wherein the microprocessor is further configured to fuzzify the signal indicative of a rate of change of wheel turning, by converting it into a first set of two values indicative of membership in two fuzzy sets, fuzzify the signal indicative of an error in wheel steering position by converting it into a second set of two values indicative of membership in two fuzzy sets, convert the first and second sets into a third set of values indicating membership in two or more fuzzy steering command sets by a fuzzy logic inference engine using a fuzzy logic rule base, and defuzzify the third set of values into the steering command indicative of the degree of opening of the valve.
The vehicle may also include a position sensor coupled to move responsively with both the wheels and the actuator and to provide the signal indicative of such motion. The signal indicative of such motion may be directly representative of the actuator position. The signal indicative of an error in wheel steering position may be derived from the signal indicative of such motion. The vehicle may also include a trajectory-planning controller configured to compute a vehicular course of travel through the field and to derive the signal indicative of an error in wheel steering position and the signal indicative of wheel steering rate.
In accordance with a third embodiment of the invention, a method for controlling the steering of a vehicle having steerable wheels steered by a hydraulic actuator, wherein the actuator is controlled by a proportional control valve regulated by a signal indicative of a degree of valve opening, the vehicle further having a microprocessor-based controller configured to generate the signal thereby controlling the steering position of the wheels, is disclosed, the method including the steps of receiving a first real-valued signal indicative of an error in wheel steering position, converting the first real-valued signal into a first plurality of values indicative of membership in a corresponding first plurality of fuzzy steering error domains, receiving a second real-valued signal indicative of a rate of wheel steering, converting the second real-valued signal into a second plurality of values indicative of membership in a corresponding second plurality of fuzzy steering rate domains, combining the first and second plurality of values to generate a third plurality of values indicative of membership in at least two of five steering command domains using a plurality of fuzzy logic rules in a fuzzy logic rule base that are selected at least to reduce the steering error signal toward zero, converting the third plurality of values into a single real-valued steering command signal indicative of the degree of opening of the valve, converting the single real-valued steering command signal into a valve signal in a driver circuit, applying the valve signal to the valve, and responsively moving the actuator to a position in which the steering error is reduced.
The step of converting the first real valued signal may include the steps of comparing the first real-valued signal with at least five fuzzy steering error domains, determining the degree of membership in each of the at least five fuzzy steering error domains, and producing at least two values indicative of the degree of membership in at least two of the five fuzzy steering error domains. The step of converting the second real valued signal may include the steps of comparing the second real-valued signal with at least five fuzzy steering rate domains, determining the degree of membership of each of the at least five fuzzy steering rate domains, and producing at least two values indicative of the degree of membership in at least two of the five fuzzy steering rate domains. The rulebase may associate the at least five fuzzy steering error domains and the at least five fuzzy steering rate domains with at least five fuzzy logic steering command domains. The step of combining the first and second pluralities of values may include the steps of accessing the rulebase with the first and second pluralities of values, determining the degree of membership of each of the at least five fuzzy steering commands, and producing at least two steering command values indicative of the degree of membership in at least two of the five fuzzy logic steering command domains. One of the fuzzy logic steering rate domains may be centered on a steering rate of zero, one of the fuzzy logic steering domains may be centered on an error of zero, and wherein one of the fuzzy logic steering command domains is centered on a command signal providing an effective flow rate of zero through the valve. There may be at least seven fuzzy logic steering rate domains and three of those domains may symmetrically balanced with three other steering rate domains about a steering rate of zero. There may be at least seven fuzzy logic steering error domains and three of those domains may be symmetrically balanced with three other steering error domains about a steering error of zero. There may be at least seven fuzzy logic steering command domains and three of those domains may be symmetrically balanced with three other steering command domains about a steering command providing an effective flow rate of zero through the valve.